In the single-color or multi-color printing of individual objects, particularly with variable data and particularly comprising at least one flat surface to be imprinted, the individual objects are moved along a production path from at least one input station, through various treatment (e.g. whitening, printing, coating, drying) stations, and are taken off the production pat at a downstream output station.
The industrial printing of such individual objects as data carriers, for example CD's or DVD's or other products used in daily life, is done by means of screen printing, flexographic printing, pad printing, offset printing, flat-bed printing, rotogravure or letterpress printing to transfer one or more colors to an object surface to be printed. The imprint serves decorative, advertising and/or informational purposes, either for the end user or for product tracking by the manufacturer.
In addition, increasingly efforts are in progress to use the image printed on an object as an individualization instrument or for product protection purposes, for example in that variable data is imprinted in plain text or as machine-readable code. In this way, for example, an imprint on a data carrier, such as a CD or DVD's, may comprise a variable serial number or an access code in addition to an otherwise non-variable background. For this purpose, either separate identifying methods or additional digital printing methods, such as ink-jet printing, laser printing, transfer printing and the like, are used, so as to apply individualized printed data.
In order to achieve full individualization, which allows the complete layout, for example of a series of CD's to be printed, to be optionally varied within the production sequence, CD printing systems are equipped, for example, with computer-controlled printers that make it possible to provide the entire surface of the CD to be printed with a multi-color imprint.
Similar methods and printers are known from home and office applications, where office printers today are able to print CD's bearing elaborate printing. The print quality approaches that of offset print.
All these printing methods that are already in use share the fact that they print the objects in linear sequence, each object running sequentially through a certain number of treatment stations. The treatment stations may comprise handling devices, test and readers, substrate pre-treatment, printing stations, coating stations, drying stations and the like. Every object being printed goes through every station, although some stations may not act on some objects or may treat different objects differently.
The material flow of the objects through all treatment stations present in a certain printing system means each object at any given time is followed by another object or at least an object position, thus creating a linear processing chain in which each object successively passes through the consecutive treatment stations. A malfunction interrupting this chain at any point results in stoppage of the entire production run until the error has been eliminated and production can resume. The result is a considerable loss of actual production.
Experience has shown that ink-jet print heads are particularly susceptible to problems, triggered either by external influences, such as vibration, high acceleration, dust, electrostatic charge, mechanical impact and the like, or by internal influences, such as ink contamination, ink outgassing and the like, as a result of which individual or a plurality of nozzles or nozzle groups or even entire print heads can fail. Since the print heads used in these printing methods operate based on the drop-on-demand principle, each head has an array of individually controlled nozzles, so that the failure of one or more nozzles of a print head fail causes undesirable stripes in the printed image or dropping out individual colors partially or completely. Automatic or manual cleaning and restart of the affected print heads in general eliminates the defect, however for the duration of the cleaning process this print station of the entire printing system is down, production is stopped. This lowers the effective production output considerably, as mentioned above.
An increase in the effective production output in this configuration of the printing system may therefore only be achieved by considerably improving the reliability of the print heads and/or by considerably increasing the speed of the printing operation and all other treatment stations so as to compensate for the down time. Even simultaneous printing of multiple objects in a common printer offers effective improvement only part of the time. When the current print speed increases, in the event of an error not only one object will be printed incorrectly, but a certain quantity, considerably increasing the volume of waste. In addition, increased print speed of the digital printers is associated with the risk of more frequent failures because all components involved in the process are subject to greater use and stress.